| Since traditional AB5-type hydrogen storage alloys are restricted from low capacity and relatively high price, Mg-based alloys, seem to be good choices due to their advantages of economy and high hydrogen storage capacities. Hence, it is significant to enhance their hydriding/dehydriding kinetics and improve the cycling stability. This paper presents a novel preparation technique named rapid quenching in magnetic field to modify the alloys. X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) were used to study the solidification behavior of the Mg-based alloys affected by external magnetic field during rapid quenching. The interaction of microstructure, morphology and surface configuration as well as component homogeneity of the alloy melts with the magnetic field was investigated. Combined with electrochemical measuring technology, the relationship between microstructure and electrochemical performance was also learned.Mg2Ni alloy was rapidly quenched with and without magnetic field. The results show that the applied magnetic field leads to the generation of directional columnar crystals during the rapid solidification of alloy melt, which have profound effect on the electrochemical properties. Meanwhile, decreased grain size and increased internal strain are noted for the magnetic field treated alloy, as well as the eliminated composition segregation. It is found on the charge-discharge experiments that the as-prepared alloy displays an increased capacity of142.05mAh-g-1with capacity retention ratio of56.5%after15cycles, which are71.4mAh·g-1and26.7%, respectively, for the ca-cast alloy. The potentiodynamic polarization results indicate that the Mg2Ni alloy with columnar structures exhibits relatively high corrosion resistance against the alkaline solution. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) results demonstrate enhanced electrochemical kinetics for the treated Mg2Ni alloy.Mn doping Mg2Nio.8Mn0.2hydrogen storage alloys was rapidly quenched in the presence of an external magnetic field. It shows that the transversal static magnetic field can effectively refine the grain size from XRD and SEM results. The Mg2Nio.8Mno.2alloy has the grain size of about200nm at the quenching rate of30m·s-1. However, the corresponding grain size for the magnetic field treated alloy at the same rate is decreased to50nm. This distinct phenomenon is probably attributed to the Lorentz force suppressing the crystallization of the alloy and the thermoelectric effect. Mainly due to the grain refinement, the discharge capacity of Mg2Nio.8Mno.2alloy is raised from79to192mAh·g-1. It is confirmed that Mg2Nio.8Mno.2alloy prepared by magnetic field assisted approach possesses lower charge-transfer resistance, enhanced electrocatalytic activity and higher hydrogen diffusion coefficient compared with the alloys without magnetic field treatment by EIS, CV and constant potential step measurements. In addition, relatively high corrosion resistance against the alkaline solution is found for the as-prepared alloy.(Lao.67Mg0.33)2Ni7alloy was rapidly quenched with and without magnetic field. XRD results show that the grain size is decreased by the affection of magnetic field. The lattice parameters are minished after rapid quenching. While the magnetic filed results in the increase of lattice parameters, as well as the c/a ratio. Phase transformation from (La,Mg)Ni5to (La,Mg)2Ni7is found after rapid quenching whether or not the magnetic field is imposed. The charge-discharge experiments show that alloy by rapid quenching only is promoted in cycling stability but decreased in capacity. As for the magnetic field treated alloy, both the discharge capacity and cycling stability are improved. Additionally, it is found that hydriding/dehydriding kinetics and high rate property are elevated by magnetic field treatment, and the charge-transfer resistance is decreased. |
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